Which of these numbers is prime? ${19,\ 35,\ 38,\ 39,\ 57}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 19 are 1 and 19. The factors of 35 are 1, 5, 7, and 35. The factors of 38 are 1, 2, 19, and 38. The factors of 39 are 1, 3, 13, and 39. The factors of 57 are 1, 3, 19, and 57. Thus, 19 is a prime number.